The present invention generally pertains to motor vehicles. More particular, the present invention pertains to a method of controlling a transmission. More specifically, but without restriction to the particular embodiment and/or use which is shown and described for purposes of illustration, the present invention relates to a method for controlling a transmission having a dual clutch system during vehicle launch.
There are presently two typical power transmissions in use on the conventional automobile. The first, and oldest, type of powertrain is the manually operated powertrain. These powertrains are characterized by having manual transmissions including a clutch pedal to the left of a brake pedal and a gear shift lever which is usually mounted at the center of the vehicle just behind the dashboard. To operate the manual transmission, the driver must coordinate depression of the clutch and accelerator pedals with the position of the shift lever in order to select the desired gear. Proper operation of a manual transmission is well known to those skilled in the art, and will not be described further herein.
In a vehicle having an automatic transmission, no clutch pedal is necessary. The standard H configuration of the shift lever is replaced by a shift lever which typically moves back and forth. The driver need only select between park, reverse, neutral, drive, and one or two low gears. As is commonly known in the art, the shift lever is placed in one of several positions having the designator P, R, N, D, 2, and maybe 1 which correspond to Park, Reverse, Neutral, Drive, and one or two low gears, respectively. Vehicle operation when the gear shift lever is placed in one of these positions is well known in the art. In particular, when in the drive mode, the transmission automatically selects between the available forward gears. As is well known, older systems typically included first, second and third gears, while newer systems include first through third gears as well as a fourth and possibly a fifth and a sixth overdrive gears. The overdrive gears provide an improved fuel economy at higher speeds. As is well known, early transmissions were almost exclusively manually operated transmissions.
With a steady development of automatic transmissions, drivers increasingly gravitated toward the easy operation of automatic transmissions. However, in the mid 1970s, rising concerns about present and future fossil fuel shortages resulted in an implementation of corporation average fuel economy (CAFÉ) regulations propagated in several countries. These fuel economy requirements necessitated the investigation of increasing the fuel economy of motor vehicles in order to meet government regulations. These government regulations prompted a gradual return to manual transmissions which are typically more efficient than automatic transmissions.
In the ensuring years, many mechanically operated vehicle systems were replaced or at least controlled by electronic control systems. These electronic control systems greatly increased the fuel efficiency of vehicle engines and enabled a gradual return to the convenience of automatic transmissions. In addition, electronic controls used with automatic transmissions, greatly improved the shift schedule and shift feel of automatic transmissions and also enabled implementation of fourth and fifth overdrive gears thereby increasing fuel economy. Thus, automatic transmissions have once again become increasingly popular.
Automatic and manual transmissions offer various competing advantages and disadvantages. As mentioned previously, a primary advantage of a manual transmission is improved fuel economy. Conversely, automatic transmissions first and foremost offer easy operation, so that the driver need not burden both hands, one for the steering wheel and one for the gear shifter, and both feet, one for the clutch and one for the accelerator and brake pedal, while driving. When operating an automatic transmission, the driver may have both one hand and one foot free. In addition, an automatic transmission provides extreme convenience in stop and go situations, as the driver need not worry about continuously shifting gears to adjust to the ever-changing speed of traffic.
The primary reason for the superior efficiency of the manual transmission over the automatic transmission lies in the basic operation of the automatic transmission. In most automatic transmissions, the output of the engine connects to the input of the transmission through a torque converter. Most torque converters have an input impeller that is connected to the output shaft of the engine and an input turbine that is connected to the input shaft of the transmission. Movement of the impeller at the input side results in a hydraulic fluid flow which causes a corresponding movement of the hydraulic turbine connected to the input shaft of the transmission. While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the powertrain. Further, the shift operation in an automatic transmission requires a hydraulic pump which pressurizes a fluid for clutch engagement. The power required to pressurize the fluid introduces additional parasitic losses of efficiency in the powertrain.
Before a shift between the gear ratios of a manual transmission can occur, it is necessary to synchronize the rotational speed of the driveshaft with the rotational speed of the driven shaft. Typically, synchronization is obtained in a manual transmission by way of a synchronizing mechanism such as a mechanical synchronizer which is well known in the art. The mechanical synchronizer varies the speed of the driveshaft to match the speed of the driven shaft to enable smooth engagement of the selected gear set. For example, during an upshift, the mechanical synchronizer utilizes frictional forces to decrease the rate of rotation of the driveshaft so that the desired gear of the driveshaft is engaged smoothly to drive the desired gear of the driven shaft. Conversely, during a downshift, the mechanical synchronizer increases the rate of rotation of the driveshaft so that the desired gear is engaged smoothly to drive the desired gear on the driven shaft.
Typically, with a manual transmission, there is a delay period between disengagement of the currently engaged gear and the subsequent synchronization and engagement of the desired transmission gear. Also, during this process, the clutch connection between the engine output shaft and the transmission input shaft needs to be disengaged prior to the gear shifting process and reengaged upon synchronization. These delays and periods of clutch disengagement create periods of torque interruption that are generally undesirable and usually result in a noticeable jolt as the gears are shifted. Such a jolt is particularly noticeable in the shift between first and second gears as the vehicle accelerates.
In order to reduce these jolts and to still take advantage of the benefits of manual transmissions, as well as to provide an automated shifting system, various designs have been proposed. In particular, various dual clutch manual transmissions have been proposed that include automated electromechanical shifting mechanisms and methods. For example, U.S. Pat. Nos. 6,044,719 and 6,012,561, which are incorporated herein by reference, each disclose a dual clutch electo-mechanical automatic transmission.
In general, these dual clutch type systems attempt to reduce the jolt associated with torque interruption as gears are shifted by starting to engage the next gear with one clutch while the current gear is disengaged with the other clutch. To further reduce the jolt associated with gear shifts in these types of transmissions, methods to control dual clutch transmissions have also been proposed.
For example, U.S. Pat. Nos. 5,950,781 and 5,915,512 each disclose a twin-clutch transmission having two input shafts and a method for controlling the transmission. The first input shaft is attached to the primary drive gears, and the second input shaft is attached to one or more auxiliary gears. The method disclosed is for controlling a gear shift between primary gears on the first input shaft wherein an auxiliary gear on the second input shaft provides a filler torque during the change in primary gears. This method is designed to reduce the jolt associated with the primary gear changes by use of the filler torque.
Although the use of a filler gear may reduce the jolt involved with shifting from the first primary gear to the second primary gear, the filler torque method still involves changing from 1st to 2nd gear, which will include at least somewhat of a jolt due to the typically large difference in these gear ratios. Additionally, this method places a large amount of stress on the clutch associated with the first drive gear, which typically must transmit a large amount of torque to initially start the vehicle moving from a dead stop. As a result, a large amount of heat is typically generated in this clutch during vehicle launch. This clutch is even further stressed during vehicle launch when the vehicle hauls a large load.
Alternatively, U.S. Pat. Nos. 4,790,418; 4,611,698; 4,527,678; 4,519,484; 4,412,461; and 4,376,473 each disclose a method for controlling a multi-clutch transmission. Particularly, each of them disclose a method for controlling the transmission during a gear shift, and each of them teach that the clutch associated with an engaged drive gear is disengaged while the clutch associated with the next gear to be engaged is substantially and concurrently engaged. Although these methods reduce the jolt from gears being changed, they still involve the changing of gears between 1st and 2nd gear, which will include at least somewhat of a jolt due to the typically large difference in these gear ratios. Additionally, these methods place a large amount of stress on the clutch associated with first gear during vehicle launch, particularly when hauling a large load. As a result, a large amount of heat is typically generated in this clutch during vehicle launch.
In order to address these shortcomings and to generally eliminate the 1st gear to 2nd gear upshift, various methods for controlling a dual clutch transmission have been developed. However, all are associated with limitations.
It is a general object of the present invention to provide a method of controlling a transmission having a dual clutch system.
It is another object of the present invention to provide a method of controlling a motor vehicle transmission which substantially eliminates torque interruption associated with shifting between first and second gears.
It is another object of the present invention to provide a method of controlling a motor vehicle transmission that allows the vehicle to be launched with the transmission in a position to selectively provide for maximum power or greater fuel economy depending on vehicle loading or launch inertia.
It is another object of the present invention to dissipate heat generated in a transmission during launch of a vehicle through two clutch assemblies, thereby resulting in lower temperatures and greater clutch assembly durability.
In one form, the present invention provides a method of controlling a dual clutch transmission of a motor vehicle, wherein the first clutch acts to transmit torque to the first driven gear, and the second clutch acts to transmit torque to the second driven gear. The steps involved in controlling the transmission include the following: determining a predetermined first clutch slip value based on the perceived vehicle loading, initiating launch of the motor vehicle with both the first and the second clutch partially engaged, determining the vehicle inertia value based on the summation of instantaneous vehicle inertia values during vehicle launch, and controlling either the first or the second clutch to disengage when the predetermined first clutch slip value is reached.
The perceived vehicle loading is preferably determined at the outset based on the overall mass of the vehicle and occupants therein, although it may also be based on the perceived slope of the ground. Alternatively, the step of determining the first clutch slip value may occur after the vehicle launch is initiated, and the perceived vehicle loading may be based on either a series of instantaneous inertia measurements or an evaluation of the amount of engine torque transmitted.
The step of determining the vehicle inertia value preferably occurs during vehicle launch, although it could be based on a determination made before vehicle launch. The vehicle inertia value is preferably determined based on a summation of the instantaneous vehicle inertia values determined during vehicle launch, and each of the instantaneous vehicle inertia values are preferably based on a comparison of an instantaneous engine speed and the corresponding instantaneous vehicle speed.
The predetermined first clutch slip value is preferably a function of the perceived vehicle load. The perceived vehicle load is preferably determined from vehicle load measurements; however, it could be determined from other factors, such as a comparison of engine torque with vehicle speed during vehicle launch. The predetermined first clutch slip value is preferably determined by comparing the speed of the engine flywheel and the speed of the transmission input shaft associated with the first clutch; however, it could be determined in other ways such as by monitoring the position of an electromechanical first clutch actuator. When the predetermined first clutch slip value is reached, the step of controlling either clutch to disengage is preferably performed based on the vehicle inertia value. In order to accomplish this step, the transmission controller preferably evaluates the vehicle inertia value to determine whether the vehicle needs maximum power, in which case the controller directs the second clutch to disengage and the vehicle continues launching in the first driven gear, or whether it is appropriate to preserve fuel economy, in which case the controller directs the first clutch to disengage and the vehicle continues launching in the second driven gear.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.